Dubai could now be the riskiest place in the world for the unwary traveller and Venezuela and Trinidad the countries with the most dangerous prisons. Those are the warnings of Prisoners Abroad (PA), which this month marks its 30th anniversary as the organisation that helps Britons who fall foul of the law abroad.

Some people setting off on their holidays this summer will be joining the increasing number of travellers locked up abroad. In the past year, more than 1,600 Britons have found themselves in foreign jails, from Vietnam to Brazil and Pakistan to Ghana. Around half were jailed for drugs offences, said Pauline Crowe, chief executive of PA. "They cover the full range from those who were 'carrying a bag for a friend' to those who admit they were involved," said Crowe. "You also get young women who have fallen in love with someone and agree to carry drugs for them."

While drug offences account for 47% of all prisoners, murder (14%), violence (9%), sexual offences (9%) and fraud (7%) also feature. Seven per cent of the prisoners are under 25 and 8% over 60. Men account for 87% of those in jail.

The United Arab Emirates has emerged as the country with the longest list of banned substances, which include many commonly available drugs, such as codeine, and many well-known anti-depressants, which can lead to jail if unaccompanied by a doctor's written prescription. Dubai airport has recently installed new drug-sensitive equipment which can detect a trace of a drug on a shoe. Earlier this year, Fair Trials International (FTI) published a list of the drugs that could lead to terms of imprisonment in Dubai. Travellers carrying jet-lag medicine have been held. Even poppy seeds can result in a charge. Fifty-nine Britons were arrested in Dubai last year on drug charges.

"Ignorance is no defence," said Crowe. "It is very easy to come unstuck. The best advice is be sensible and do your research before you travel." In February, the BBC DJ Grooverider was jailed for four years for an amount of cannabis that would not have led to his arrest in the UK.

The US has the largest number of UK prisoners, followed by Spain, France, Australia and Thailand, where there are 45 British men in jail. Venezuela and Trinidad were regarded as having the toughest prisons. "It's absolutely lawless in Venezuela," said Crowe. "The prisoners are better armed than the guards." Deaths in foreign prisons are not uncommon. Earlier this year, a Briton died in jail in Laos from illness and starvation. The prisoner, jailed for money laundering, had mental problems and refused treatment.

Around 70 prisoners have been transferred back to Britain in the past year but others prefer to stay in jails abroad. "In Thailand, for instance," said Crowe, "you might be in a cell with 30 or 40 other prisoners but you have free association throughout the day, if you can cope with the weather and the insect infestation. In Britain, the Prison Service is so overstretched that you could be locked up for most of the day."

Joe Parham, co-founder of what was originally called the National Council for the Welfare of Prisoners Abroad, said that when it started in 1978 there had been a predominance of people arrested in Spain. "We had a huge number coming up from Morocco with drugs who were caught like flies at Algeciras." Travellers returning from places such as Thailand and Turkey brought in tales of friends serving long sentences, mainly for drugs.

The map of prisoners abroad is due to change dramatically in 2010 when the Transfer of Sentenced Prisoners Act comes into force. This will mean than any prisoners from an EU country will be sent back to their home country to serve their sentence.